Lubricating oils have been conventionally used in internal combustion engines and automatic transmissions for making them work smoothly. Particularly, lubricating oils for internal combustion engines (engine oils) have been required to have high characteristic performances due to the fact that recent developed engines have been improved in performances, increased in power output and used under more sever operation conditions. Therefore, conventional engine oils are blended with various additives such as anti-wear agents, metallic detergents, ashless dispersants, and anti-oxidants for fulfilling the required performances. Since fuel efficient engine oils are large in energy loss at friction parts of engines where the oils are involved, sulfur-containing organic molybdenum compounds such as molybdenum dithiocarbamate and molybdenum dithiophosphate have been preferably used as measures for friction loss or fuel efficiency decrease because these compounds exhibit excellent friction reducing effects. In order to make these compounds fully exhibit such friction reducing effects, a method has generally been employed wherein the compounds are used in combination with a sulfur-containing compound such as zinc dithiophosphate in a rather larger amount such that a molybdenum disulfide film is formed on sliding surfaces of an engine. Furthermore, it is generally known that metallic detergents such as salicylate-based detergents can exhibit friction decreasing effects more with the increase of the amount. However, it has not been fully studied to decrease the metal (decreased ash content) and sulfur contents of conventional low friction lubricating oils such as fuel efficient engine oils. It has been realized that sulfur-containing compounds as described above are significant in effects to improve oxidation stability while they invite the depletion of organic molybdenum compounds or metallic detergents under the influence of sulfuric acid resulting from the deterioration or decomposition of themselves and thus that there is a limit to maintain the initial low friction characteristics for a long period of time. That is, conventional techniques had a limit to provide an engine oil which has excellent low friction properties and can achieve low ash and sulfur contents and improved long-drain properties more than before.
On the one hand, lubricating oils have been required to be decreased in not only ash and sulfur contents but also phosphorus contents so as to reduce harmful influences on recent-developed exhaust-gas purifying catalysts such as ternary catalysts, NOx adsorbers, and oxidation catalysts or exhaust-gas after-treatment devices such as exhaust gas recycle devices (EGR) and diesel particulate filters (DPF) as much as possible.
The present invention was made in view of the foregoing requirements and intends to provide a lubricating oil composition having both low friction properties and long-drain properties such as oxidation stability and base number retention properties, particularly suitable for internal combustion engines.